Nightmare

Nightmare is a bio-weapon that serves its creator, Mundus. It is the boss of Mission 16: Nightmare of Darkness, Mission 18: Spirit Stone, "Elixir" and Mission 20: Showdown with Nightmare in Devil May Cry. There are two weapons fashioned after it: Nightmare-β, which appears in Devil May Cry; and Nightmare-γ in Devil May Cry 2.

Appearance and Description
The Nightmare takes the form of a slug-like demon which has many huge metallic parts embedded and making it up, there is also several skulls giving it its "Nightmarish" look, which strikes fear into it. It is forged from the darkness of a small mirror-like puddle giving its initial "Non-Existence".

It was created by Mundus out of inorganic substances, and it is unknown whether it is even conscious. It is powered by several cores which it rarely exposes and which can absorb Devil Trigger power from Dante; if it is allowed to absorb enough, it can activate its own Devil Trigger. It also possesses a multitude of offensive mechanisms, and it will change its attack pattern each time one of its cores is destroyed. It generally exists in an invulnerable gel-like form which includes the remains of its victims, and in this form it can swallow enemies and teleport them to an evil dimension in which their trauma manifests as evil spirits. In order to be damaged, it must be forced into a more solid form by activating embossed discs on the ground, These are restraining tools used to keep it from overriding. However, in this form it will encase itself in a hard armor, and it will attack more viciously.

Story
Dante first encounters the Nightmare within the castle cathedral, but he forces it to retreat. It later appears while Dante performs the ritual to transform the Philosopher's Egg into the Elixir, and is again forced to retreat. After Dante enters the Underworld, he is lured into the Nightmare's den by Trish, who is pretending to be injured. However, Nightmare attacks as soon as Dante enters the room, and while fighting it, Dante is also attacked by Trish. Dante manages to finally destroy Nightmare, and coldly leaves Trish in the room while he goes on to fight Mundus.

Strategy
Nightmare is invulnerable to all attacks except for its core. To expose the core, shield devices located within the battlefield must be activated to bathe the room in light. This turns Nightmare from a puddle into a solid state, and it will expose its core during certain attacks. When the lights go out, Nightmare turns into a puddle of goo and can envelop Dante whole, transporting him into its insides where Dante must do battle with one of the other bosses of the game, Phantom, Griffon, or Nelo Angelo, in order to escape.

Powers and Abilities
Nightmare is a high-tech demon, armed with various weaponry. Its demonic physiology was sensed by Dante in the ooze which is basically a part of it. Nightmare is also an Ambush Predator, designed to disguise itself in the surroundings and eventually attack its prey, if they're foolish enough to come near it. Once it's detected, this enemy shows itself as an enormous slug-based demon with a size of a bus and having a metallic exterior and small skulls in its ooze that is meant to strike fear into any threat, or something else more haunting. Its weapons include a barrel-designed face that produces large and potent laser waves, copy its design to send miniature Nightmares that will attempt to freeze the player, throw an automated boomerang-esque blade that is very difficult to avoid, has many mid-ranged weapons that hover and fire off sharp energy shots and can even adapt to move quick and revert to its goop form to cling at the wall and ensnare whatever is underneath in a demonic netting-looking cage, or use its own form to swallow and transport them into a mindscape that reflects whoever fears the most unless its victim manages to break free which will damage it greatly.

As for its physical abilities, Nightmare is vulnerable to artificial light and its core that is randomly placed, is a weakling, it depends on a variety of its weapons to survive but is very fast in exchange to these drawbacks. However, with everything shown, Nightmare is one of the most overwhelming demons anybody can face, not just Dante.

Trivia

 * Nightmare means "a person, thing, or situation that is very difficult to deal with." which is happen to be what the demon Dante faces upon.